The measurement of transformation kinetics and microstructural observation of bainite have been performed using Fe-9%Ni-C and Fe-2%Si-I%Mn-C alloys in order to clarify the formation mechanism of the ferrite component of bainite in steels. The main findings are as follows.(1) In all of three Fe-9%Ni-C alloys with different carbon contents from 0.1% to 0.3%, the TTT diagram consisted of two C-curves ; a higher one corresponded to pro-eutectoid ferrite + pearlite and a lower one corresponded to bainite. This clearly indicates that the bainite reaction has its own C -curve even in the alloys without strong carbide formers. Moereover, it was confirmed that the Bs temperature was almost independent on the carbon content and corresponded to the Ms temperature of the Fe-9%Ni binary alloy.(2) In the 2%Si-I%Mn-0.34%C hypoeutectoid steel, carbide-free bainitic ferrite formed predominantly at 823K, even though the amount of proeutectoid ferrite decreased with decreasing the decomposition temperature and the structure became fully pearlitic at 873K. Moreover, in the 2%Si-I%Mn0.59%C eutectoid steel, carbide-free bainitic ferrite formed at temperatures from 823K to 723K, whereas the structure was fully pearlitic at higher temperatures between 973K and 873K. These results indicate that the formation mechanism of bainitic ferrite is different from that of the proeutectoid ferrite.(3) TEM observation of bainite formed from work-hardened austenite in the Fe-2%Si-I%Mn-0.59%C alloy showed that the bainitic ferrite inherited the dislocation cell structure from the work-hardened austenite. On the other hand, dislocations were scarcely observed in pealite formed from the work-hardened austenite.It was concluded from the above results that the bainitic ferrite forms by the displacive mechanism rather than the diffusional one.